A Gene Map for the Cute Side of the Family

When scientists announced last week that they had deciphered the complete genetic playbook for the duck-billed platypus, the public reacted with considerably more enthusiasm than it had accorded similar bulletins about the sequencing of, say, the mustard plant, the mosquito or the wild chicken. A “fantastic response,” said Jennifer Marshall Graves of the Australian National University in Canberra, a principal author on the report. “More than I expected.”

One reason for the glowing reviews is that people love platypuses the way they love penguins and panda bears, as adorable, clumsy and nonthreatening creatures that remind them of kids playing dress-up. But the platypus trumps its plush-toy costars by adding a kind of Dada prankishness to the equation, what with its bill that looks like a Charlie Chaplin shoe, the leathery, thumb-sized eggs it insists on laying, the Daffy Duck webfeet outfitted with venomous spurs and the milk that dribbles down its unnippled chest. That the genetic code of the platypus proved to be as bizarrely pastiched as its anatomy enhanced the popular appeal of the report, published in the journal Nature.

Yet for researchers in the burgeoning field of comparative genomics, the real beauty of having spelled out all 2.2 billion chemical letters of the platypus’s genetic blueprint lies not with the freak-show charm of the animal but rather with its sublime ordinariness, positioned as almost a platonic abstraction of a mammal, yet one with enough specifically derived features to remind us that it is just an animal trying to make its way in the world. It is archaic and post-modern, primitive and refined. By studying the platypus and its close relatives, scientists hope to better understand the genesis and evolution of the entire mammalian family tree.

“Modern mammalian diversity is enormous,” said Zhe-Xi Luo, curator of vertebrate paleontology at the Carnegie Museum of Natural History in Pittsburgh, “but our view of that diversity has been heavily influenced by our focus on two of the three groups of mammals, the placentals and the marsupials.”

That fixation isn’t surprising, he said, given their numbers. Placental mammals that gestate their young internally, as do we and most of the pelts we know, account for 95 percent of the world’s 5,600 species of mammals, and marsupials like the kangaroo, koala and our very own opossum, whose young are born at a grublike stage and do most of their developing externally, often in a pouch, constitute nearly all the rest.

The third group of mammals, the monotremes, claims a measly three distinct animals, and all are indigenous to Australia and New Guinea: the duckbilled platypus and the long-beaked and short-beaked echidnas, also known as spiny anteaters. Yet this small club goes very deep, and with the sequencing of the platypus genome, Dr. Luo said, “the total scope of mammalian diversity” can now be explored.

The monotremes are considered the most ancient mammalian group, dating back to the Jurassic period, which began roughly 200 million years ago. They are not our direct ancestors, but rather split off from the rodenty line that gave rise to us and marsupials some 180 million years ago, with marsupials breaking away maybe 40 million years later.

However ancient the rupture, the last common ancestor between us and monotremes was clearly mammalian, for we all sport mammalian traits, some of us more luxuriously than others. The duckbilled platypus, for example, is jacketed in two layers of fine, dense fur, the better to keep it warm in its semi-aquatic existence, and “it’s the softest fur you can imagine,” said a co-author of the platypus paper, Wesley Warren of Washington University School of Medicine.

Monotremes also have a four-chambered heart, a mammalian jaw hinge and a set of tiny middle ear bones that, unlike in reptiles, are separated from the lower jaw and hence lend mammals their highly sensitive hearing. And though a platypus mother lacks nipples, the fluid that oozes from two round patches of skin on her belly is the same sort of rich blend of sugars, proteins, fatty acids, vitamins and antimicrobial agents lionized by La Leche.

Monotremes also possess features that hark back to a runty mammal’s lot at play among the giants of Jurassic park. They can dig themselves into the haven of a hole within moments. They are most active in the evening and at dawn, in contrast to the diurnal dinosaurs. They have true or vestigial spurs on their back legs, which the male platypus still packs with a snakelike venom. While the poisonous spearlets are now directed largely against rival males, the trait could well have served among ancestral monotremes as the equivalent of a bee’s sting, to ward off the Bigfeet that ever threatened to crush them.

At the same time, the monotremes still pay homage to their premammalian forebears. The group name, monotreme, means “one-holed,” and so echidnas and platypuses have the equivalent of a bird or reptile cloaca, a single all-purpose orifice for excretion, sex and the laying of eggs.

The laying of eggs is, of course, a monotreme’s clearest vinculum to a reptilian-avian mode of life. After digging herself an underground nest and padding it with vegetation, a female platypus lays one to three eggs and incubates them for about 10 days.

On hatching, the babies are barely the size of a gumdrop, and they will spend the next five months nursing in the burrow, emerging only when they are close to full-grown, maybe the size of a housecat. It’s time to start hunting for a living, to dive into a pond in search of crustaceans, which the platypus does with the help of chemoreceptors to smell the prey and electroreceptors to detect the minor electrical field surrounding the prey and mechanoreceptors to track movement of the prey and a toothless but tough horny bill for seizing and crushing the prey it has amply sensed.

For its part, the echidna is highly specialized for eating ants and termites, with a long pointed snout and a wormlike, sticky tongue.

In the specifics of their hunting gear, the platypus and echidna reveal their highly stylized sides, the kidskin fit between themselves and their niches, and the ways they have profoundly evolved since their ancestors and ours started loosening their jawbones and sweetening the glandular secretions with which they moistened their eggs. Monotremes are like the fantasy geezer aunt you never knew you had, breezing in from the other side of the world, with wild tales of the past and no plans for slowing down.

A version of this article appears in print on , on page F1 of the New York edition with the headline: A Gene Map for the Cute Side of the Family. Order Reprints|Today's Paper|Subscribe